The present invention relates to a mechanical seal for a compressor and to a centrifugal compressor comprising the mechanical seal.
A conventional mechanical seal which is used for providing a seal between a casing and a shaft of a centrifugal compressor will be explained by means of FIG. 7 which shows the cross-sectional view of the main part of the conventional mechanical seal.
Two annular rotating slide members 33 are attached to an annular support member 32 by shrinkage. The outer end surface 36 of each slide member 33 forms a sliding surface which contacts an opposed stationary slide member 34 exerting an appropriate pressure on the rotating slide member 33. The rotating slide member 33 and the stationary slide member 34 prevent fluid from leaking out of a centrifugal compressor at the sliding area. The outer diameter of the rotating slide member 33 is constant.
In order to determine the appropriate load for shrinkage fitting the stiffness of the materials of both slide members are to be taken in consideration. The slide members are required to have low friction at the slide surfaces thereof. Because a corrosive gas such as hydrogen sulfide is often dealt with, the slide members have to be resistant against corrosion. Finally the wear has to be small so that the slide members can be used for a period of at least 2 to 3 consecutive years.
For the above mentioned reasons hard materials such as the SiC (silicon carbide) are appropriate for the slide members. However, silicon carbide is a brittle material so that when an excessive load is applied to the slide members, cracks may occur at the surface of the slide members.
On the other hand, after shrink-fitting the rotational slide member 33 and the support member 32 of the conventional mechanical seal of FIG. 7, the rotational slide member 33 is supported at its periphery at the edge 38 of an inwardly extending fitting portion 37 of the support member 32. Therefore, the shrinkage stress acts at an extremely small portion of the periphery of the rotational slide member 33. This causes a stress concentration at a contacting area of the rotaional sliding member 33 and the support member 32 which can generate a cracks on the outer surface of the rotating slide member 33.
In order to reduce the stress concentration it can be considered to round the edge 38 of the fitting portion 37 of the support member 32 as shown in FIG. 6.
However, the rounding of the edge 38 does not prevent the rotating slide member 33 from displacing in axial direction. It is further necessary to consider the additional radial stress which is caused by centrifugal forces.
The shrinkage stress becomes greater when the centrifugal compressor is operated at a higher speed. As the maximum shrinkage fitting load may not exceed a certain value, the shrinkage fitting load for installation has to be below a proper value.
Therefore, an axially displacement or falling out of the rotating slide member 33 may occur due to a lack of supporting force when an impact force is applied to the centrifugal compressor from outside during operation or installing thereof.
Further, when the above-described slide members are used in centrifugal compressor having a rated rotational speed of 5000-20000 rpm, a shaft power of 300-8000 kW, a shaft diameter of 100-200 mm and a seal pressure of 0.5-2 MPa, the peripheral speed of the supporting portion becomes large and the supporting member 32 may deflect by the centrifugal force. Therefore, a radial outward displacement of the rotating slide member 33 occurs. This displacement causes a decrease of the amount of interference between the rotating slide member 33 and the support member 32 during operation.
To overcome this inconvenience, it is needed to provide a larger amount of interference for shrinkage fitting in advance.
In the Japanese Utility model laid-open 5-3737/1993, a mechanical seal made of ceramics is described. Though this seal has a face plate having a shoulder made of ceramics, it merely shows protection of debris coming into the seal region and does not show details how to use in a centrifugal compressor.
It is the problem underlying the present invention to provide a mechanical seal which eliminates the above-mentioned inconveniences of the prior art, can be manufactured or processed easily and has a high reliability.
Further a centrifugal compressor with a high speed and a high reliability seal is to be provided.
This problem is solved by a mechanical seal for a compressor comprising a support member fixed to a rotary shaft for rotating therewith, an annular rotating slide member consisting of hard materials and being fitted to the support member by shrinkage; and a stationary slide member exerting pressure against an end surface of said rotating slide member; wherein said rotating slide member comprises at least one peripheral recess formed by a portion of the periphery of said rotating slide member having a smaller outer diameter than the rest of the periphery; and said support member comprises an inwardly extending fitting portion engaging the peripheral recess.
Advantegeously at least one of the edges of the fitting portion is rounded.
When the mechanical seal is used for a centrifugal compressor having a rotating shaft with a diameter of 100-200 mm, a rated rotational speed of 5000-20000 rpm, a shaft power of 300-8000 kW and a seal pressure of 0.5-2 MPa, the outer diameter of the bottom of the peripheral recess is preferably smaller than the outer diameter of the rest of the periphery by 0.01-0.1 mm and/or the rounded edge of the fitting portion preferably has a radius of curvature of 0.05-0.2 mm.
In a preferred embodiment said rotating slide member has an increased heat transfer area.
Conveniently said rotating slide member is made of silicon carbide.
The mechanical seal of the present invention has a high reliabilty and a simple construction. Therefore it is easy to manufacture.
A centrifugal compressor operated at a rated rotational speed of 5000-20,000 rpm and a seal pressure of 0.52 MPa and including a rotating shaft having an outer diameter of 100-200 mm preferably comprises a mechanical seal of the present invention to have a high reliability.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.